1. Field of the Invention
This invention relates generally to seismic exploration, and, more particularly, to harmonic noise attenuation in correlated sweep data in seismic exploration.
2. Description of the Related Art
Seismic exploration is widely used to locate and/or survey subterranean geological formations for hydrocarbon deposits. A survey typically involves deploying one or more vibrators, such as a thumper, and one or more seismic sensors, such as hydrophones or geophones, at predetermined locations. For example, a thumper may be used to drive an acoustic wave into the ground. The acoustic wave may be reflected by subterranean geologic formations and propagate back to one or more geophones. The geophones receive the reflected waves, which are then processed to generate seismic data. Analysis of the seismic data may indicate probable locations of geological formations such as hydrocarbon deposits.
In traditional seismic exploration, the vibrator uses a so-called “pilot sweep”, which is generally provided by a vibrator electronics package, to emit an acoustic signal, sometimes referred to as a “sweep,” for a selected period of time. Following each sweep, the vibrator stops to allow the seismic sensors to “listen” for the reflected signal. For example, the vibrator emits the acoustic signal for about 4 seconds and then stops emitting the acoustic signal for a listening period of about 4 seconds. The seismic sensors receive and record the reflected signal, which may be cross-correlated with the pilot sweep to create a map of nearby subterranean geologic features.
Mapping a large area using single sweeps, which are each followed by a listening period may be time-consuming. For example, a single survey of a few hundred square miles typically requires several months to complete. To try and reduce the survey time, engineers have proposed a variety of techniques, including a cascaded sweep. In a cascaded sweep, the vibrator emits a plurality of sweeps with no listening period between them. After all the cascaded sweeps have been completed, the vibrator stops for a listening period. The seismic sensor receives, and records, the reflected signal during each of the sweeps and the listening period. The recorded data from the listening period and each sweep may then be cross-correlated to create a subterranean map of the surveyed area.
A slip sweep may also be used to reduce the survey time. In a slip sweep, vibrators are deployed in the survey area. A first vibrator begins emitting a first sweep and then, a preselected time later, a second vibrator begins emitting a second sweep. In one embodiment, the preselected time is shorter than the duration of the first sweep, so the second sweep begins before the first sweep has completed. After the desired number of slip sweeps is complete, the vibrators stop emitting sweeps for a listening period. As in the cascaded sweep, the seismic sensor receives, and records, the reflected signal during each of the sweeps and the listening period.
Despite the acknowledged potential to reduce the survey time, a number of technical difficulties may inhibit the widespread adoption of the cascaded sweep or slip sweep technology for seismic exploration. In particular, the vibrator generally introduces harmonic noise into the sweep. The amplitude of the harmonic noise may be comparable to or greater than the reflected signal and may thus make it difficult to separate the reflected signal from the harmonic noise. Amplitude and/or phase errors in the acoustic signal provided by the vibrator may also make it difficult to remove the harmonic noise by traditional methods, such as stacking multiple data sets. The noise degrades the quality of the cross-correlation, in some cases obscuring the geologic formations that seismic exploration is supposed to detect.